Re: [PATCH RFC 11/11] rust: sync: arc: Add lockdep integration

Gary Guo
2023-07-15T16:00:26Z

On Fri, 14 Jul 2023 18:14:03 +0900
Asahi Lina <lina@asahilina.net> wrote:

> Now that we have magic lock class support and a LockdepMap that can be
> hooked up into arbitrary Rust types, we can integrate lockdep support
> directly into the Rust Arc<T> type. This means we can catch potential
> Drop codepaths that could result in a locking error, even if those
> codepaths never actually execute due to the reference count being
> nonzero at that point.
>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
> lib/Kconfig.debug | 8 ++++++
> rust/kernel/init.rs | 6 +++++
> rust/kernel/sync/arc.rs | 71 ++++++++++++++++++++++++++++++++++++++++++++++---
> 3 files changed, 82 insertions(+), 3 deletions(-)
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index fbc89baf7de6..ff4f06df88ee 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -3010,6 +3010,14 @@ config RUST_BUILD_ASSERT_ALLOW
>
> If unsure, say N.
>
> +config RUST_EXTRA_LOCKDEP
> + bool "Extra lockdep checking"
> + depends on RUST && PROVE_LOCKING
> + help
> + Enabled additional lockdep integration with certain Rust types.
> +
> + If unsure, say N.
> +
> endmenu # "Rust"
>
> endmenu # Kernel hacking
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index f190bbd0bab1..b64a507f0a34 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1208,6 +1208,7 @@ pub trait InPlaceInit<T>: Sized {
> /// type.
> ///
> /// If `T: !Unpin` it will not be able to move afterwards.
> + #[track_caller]
> fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
> where
> E: From<AllocError>;
> @@ -1216,6 +1217,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
> /// type.
> ///
> /// If `T: !Unpin` it will not be able to move afterwards.
> + #[track_caller]
> fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
> where
> Error: From<E>,
> @@ -1228,11 +1230,13 @@ fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Pin<Self>>
> }
>
> /// Use the given initializer to in-place initialize a `T`.
> + #[track_caller]
> fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
> where
> E: From<AllocError>;
>
> /// Use the given initializer to in-place initialize a `T`.
> + #[track_caller]
> fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
> where
> Error: From<E>,
> @@ -1277,6 +1281,7 @@ fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
>
> impl<T> InPlaceInit<T> for UniqueArc<T> {
> #[inline]
> + #[track_caller]
> fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
> where
> E: From<AllocError>,
> @@ -1291,6 +1296,7 @@ fn try_pin_init<E>(init: impl PinInit<T, E>) -> Result<Pin<Self>, E>
> }
>
> #[inline]
> + #[track_caller]
> fn try_init<E>(init: impl Init<T, E>) -> Result<Self, E>
> where
> E: From<AllocError>,
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..3bb73b614fd1 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -34,6 +34,9 @@
> };
> use macros::pin_data;
>
> +#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +use crate::sync::lockdep::LockdepMap;
> +
> mod std_vendor;
>
> /// A reference-counted pointer to an instance of `T`.
> @@ -127,6 +130,17 @@ pub struct Arc<T: ?Sized> {
> _p: PhantomData<ArcInner<T>>,
> }
>
> +#[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> +#[pin_data]
> +#[repr(C)]
> +struct ArcInner<T: ?Sized> {
> + refcount: Opaque<bindings::refcount_t>,
> + lockdep_map: LockdepMap,
> + data: T,
> +}
> +
> +// FIXME: pin_data does not work well with cfg attributes within the struct definition.
> +#[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
> #[pin_data]
> #[repr(C)]
> struct ArcInner<T: ?Sized> {
> @@ -159,11 +173,14 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
>
> impl<T> Arc<T> {
> /// Constructs a new reference counted instance of `T`.
> + #[track_caller]
> pub fn try_new(contents: T) -> Result<Self, AllocError> {
> // INVARIANT: The refcount is initialised to a non-zero value.
> let value = ArcInner {
> // SAFETY: There are no safety requirements for this FFI call.
> refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + lockdep_map: LockdepMap::new(),
> data: contents,
> };
>
> @@ -178,6 +195,7 @@ pub fn try_new(contents: T) -> Result<Self, AllocError> {
> ///
> /// If `T: !Unpin` it will not be able to move afterwards.
> #[inline]
> + #[track_caller]
> pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
> where
> Error: From<E>,
> @@ -189,6 +207,7 @@ pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self>
> ///
> /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned.
> #[inline]
> + #[track_caller]
> pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self>
> where
> Error: From<E>,
> @@ -292,15 +311,46 @@ fn drop(&mut self) {
> // freed/invalid memory as long as it is never dereferenced.
> let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
>
> + // SAFETY: By the type invariant, there is necessarily a reference to the object.
> + // We cannot hold the map lock across the reference decrement, as we might race
> + // another thread. Therefore, we lock and immediately drop the guard here. This
> + // only serves to inform lockdep of the dependency up the call stack.
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + unsafe { self.ptr.as_ref() }.lockdep_map.lock();

Make the intention explicit by

drop(unsafe { self.ptr.as_ref() }.lockdep_map.lock());

and make `lock` function `must_use`.

> +
> // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
> // this instance is being dropped, so the broken invariant is not observable.
> // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
> let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
> +
> if is_zero {
> // The count reached zero, we must free the memory.
> - //
> - // SAFETY: The pointer was initialised from the result of `Box::leak`.
> - unsafe { Box::from_raw(self.ptr.as_ptr()) };
> +
> + // SAFETY: If we get this far, we had the last reference to the object.
> + // That means we are responsible for freeing it, so we can safely lock
> + // the fake lock again. This wraps dropping the inner object, which
> + // informs lockdep of the dependencies down the call stack.
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + let guard = unsafe { self.ptr.as_ref() }.lockdep_map.lock();
> +
> + // SAFETY: The pointer was initialised from the result of `Box::leak`,
> + // and the value is valid.
> + unsafe { core::ptr::drop_in_place(&mut self.ptr.as_mut().data) };
> +
> + // We need to drop the lock guard before freeing the LockdepMap itself
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + core::mem::drop(guard);
> +
> + // SAFETY: The pointer was initialised from the result of `Box::leak`,
> + // and the lockdep map is valid.
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + unsafe {
> + core::ptr::drop_in_place(&mut self.ptr.as_mut().lockdep_map)
> + };
> +
> + // SAFETY: The pointer was initialised from the result of `Box::leak`, and
> + // a ManuallyDrop<T> is compatible. We already dropped the contents above.
> + unsafe { Box::from_raw(self.ptr.as_ptr() as *mut ManuallyDrop<ArcInner<T>>) };

I feel there are a lot more `as_ref/as_mut` calls than it could be.
Could you refactor the code to make a single `as_ref()` call for the
non-zero path and a single `as_mut()` call for the zero path?

> }
> }
> }
> @@ -512,6 +562,7 @@ pub struct UniqueArc<T: ?Sized> {
>
> impl<T> UniqueArc<T> {
> /// Tries to allocate a new [`UniqueArc`] instance.
> + #[track_caller]
> pub fn try_new(value: T) -> Result<Self, AllocError> {
> Ok(Self {
> // INVARIANT: The newly-created object has a ref-count of 1.
> @@ -520,13 +571,27 @@ pub fn try_new(value: T) -> Result<Self, AllocError> {
> }
>
> /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
> + #[track_caller]
> pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
> // INVARIANT: The refcount is initialised to a non-zero value.
> + #[cfg(CONFIG_RUST_EXTRA_LOCKDEP)]
> + let inner = {
> + let map = LockdepMap::new();
> + Box::try_init::<AllocError>(try_init!(ArcInner {
> + // SAFETY: There are no safety requirements for this FFI call.
> + refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> + lockdep_map: map,
> + data <- init::uninit::<T, AllocError>(),
> + }? AllocError))?
> + };
> + // FIXME: try_init!() does not work with cfg attributes.
> + #[cfg(not(CONFIG_RUST_EXTRA_LOCKDEP))]
> let inner = Box::try_init::<AllocError>(try_init!(ArcInner {
> // SAFETY: There are no safety requirements for this FFI call.
> refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> data <- init::uninit::<T, AllocError>(),
> }? AllocError))?;
> +
> Ok(UniqueArc {
> // INVARIANT: The newly-created object has a ref-count of 1.
> // SAFETY: The pointer from the `Box` is valid.
>